Moderating the influence of a bulk material's surface chemistry, such as imparting hydrophilic nature to a hydrophobic but stable bulk material, is a technique central to many areas of research ranging from biotechnology to microelectronics. The ability to modify the nature of a surface with macromolecules has existed for some time. Methods developed allow the covalent surface attachment of pre-formed macromolecular systems, the grafting of solution initiated macromolecular chains to a suitably functionalized surface, and the surface-initiated polymerization of a variety of monomer formulations. All these methods have resulted in effective graft coating of the substrate with high grafting efficiency. However, they provide little control of the macromolecular properties (molecular weight, distribution and composition) of the polymer layer. Furthermore, as well as generating a polymer coating on the substrate, the process also results in the formation of non-bound polymer in solution, which can hinder isolation and purification of the coated substrate.
The control of polymer coatings on bulk solid-supports is key to improving the utility of combinatorial solid-phase synthesis, as well as other solid-phase applications. Solid-phase synthesis technologies are used to create large numbers of new chemical compositions across a wide range of chemical disciplines. For example, combinatorial chemical synthesis may be carried out on the solid phase to generate libraries of thousands of new chemical entities that may be evaluated as pharmaceutical lead compounds. One method for generating new useful supports for solid-phase synthesis is by grafting polymers to an underlying bulk solid support. The grafted polymers may be selected to have chemical functional groups, or other physical properties, that provide improved sites for solid-phase synthesis.
The limitations cited above have provided impetus for the development of surface-confined methods of grafting substrates of interest with predetermined macromolecular composition and architecture, employing the spectrum of commercially-available monomers susceptible to standard free radical chemistry. More recently, methods to achieve such have been reported utilizing “living”/controlled free radical polymerization (see e.g. Kato, et al., Macromolecules 28: 1721 (1995); Wang & Matyjaszewski, J. Am. Chem. Soc. 117: 5614 (1995); Pattern et al., Science 272: 866 (1996); Percec & Barboiu Macromolecules 28: 7970 (1995); Granel et al., Macromolecules 29: 8576 (1996); Hawker et al., J. Am. Chem. Soc. 118: 11467 (1996); Ejaz et al, Macromolecules 33: 2870 (2000); Mandal et al., Chem. Mater. 12: 3481 (2000); and Angot et al, Macromolecules 34: 768 (2001)). As these methods work by controlling the growth of polymer chains, the tethered chain ends are active, thus the desired polymer molecular weight and composition can be achieved by varying the nature of the monomer feed and time of polymerization. The continuous approach that these controlled methods of performing free radical polymerization allows, does not result in the generation of steric barriers imposed by crowding of the growing chains at the surface, which hinders the diffusion of chain ends to the monomer interface for further propagation of the bound macromolecules. The ability of the living free radical process to achieve these desired effects is based on the dynamic equilibrium between dormant and active chain ends. The terminology “living”/controlled radical polymerization is discussed in Darling et al., J. Polym. Sci. Part A: Polym. Chem. 38: 1706 (2000). The general features of such a polymerization are:                (1) the main chain carrier is a carbon centered radical;        (2) the control over the reaction is exerted by a reversible capping mechanism so that there is an equilibrium between dormant and active chains; this has the effect of reducing the overall radical concentration, thereby suppressing radical-radical termination events; in reversible-addition-fragmentation transfer (RAFT) polymerizations, this is achieved by a dithioester or related compound; in atom transfer radical polymerization (ATRP), this is achieved by a halogen atom and in nitroxide-mediated polymerizations (NMP) it is achieved with a nitroxide molecule;        (3) the molecular weight of the polymer grows in a linear fashion with time/conversion;        (4) “living” polymers are distinguished from “dead” polymers by having the ability to grow whenever addition monomer is supplied; and        (5) block copolymers can be prepared by sequential monomer addition.        
These general features of the state of the art of living/controlled polymerizations may be found in several recent patent publications. WO 98/01480 (Matyjaszewski et al.) describes a solution-phase method of controlled polymerization using Atom Transfer Radical Polymerization (ATRP). The disclosed method uses control agents comprising transition metal-ligand complexes. WO 97/47661 (corresponds to U.S. Pat. No. 6,310,149 B1, Haddleton) discloses additional metal-ligand complexes that are useful as ATRP control agents in solution phase controlled polymerization reactions. WO 99/28352 (Haddleton et al) discloses solid silica supports chemically modified with ligands that can be used to immobilize transition metals capable of acting as control agents for controlled polymerization reactions of the type described in WO 98/01480 and WO 97/47661. Canadian Patent application 2,341,387 (Bottcher et al.) discloses the use of a “living”/controlled polymerization method to produce defined layers of polymers on a solid surface. The disclosed “living”/controlled polymerization method requires chemically modifying the solid surface with a compound of the general formula A-L-I, where A represents an active group, I is an ATRP initiating group, and L is a linkage between them. Canadian Patent application 2,249,955 (Guillet et al.) discloses a method of graft polymerization on backbone polymers using stable nitroxide-based free-radical generating compounds. The disclosed method is limited to stable free radicals that can exist in solution for at least 24 hours without recombining with one another to any substantial extent. Moreover, the nitroxide-based free radical compounds disclosed by Canadian Patent application 2,249,955 require high temperatures (i.e. >110° C.) to achieve controlled polymerization. High temperatures during polymerization have the disadvantage of increasing the production of unbound polymerization in solution as well as causing increased “in-growth” of the graft polymer into the bulk solid support.
Notwithstanding the partial successes of previous “living”/controlled polymerization processes in the development of a range of polymers, these processes exhibit the disadvantages of requiring a multi-step approach to the attachment of the control agent to the substrate and/or high-temperature nitroxide-based control agents. Consequently, the surface of the bulk solid support must be functionalized prior to polymerization and/or treated to high temperatures that result in non-surface-bound polymers as well as undesirable polymer graft in-growth.
The in-growth, or penetration of grafts into the bulk solid support during the polymerization process is a major limitation of state of the art solid phase grafting techniques. Ideally, polymerization of grafts occurs on the outer most surface of a solid support (i.e. with little or no penetration into the bulk solid) so that functional groups on the graft have maximum accessibility to the surrounding solvent environment. The in-growth of polymer grafts results in poor accessibility to the solvent environment. Furthermore, this lack of accessibility is exacerbated by the solvent swelling that most polymers undergo during the course of a solid-phase synthesis protocol.
Consequently, the use of the polymer grafts as sites for solid-phase synthesis results in large amounts of impurities. These impurities must be separated from the desired products upon cleavage from the support resulting in lower solid phase synthesis yields and increased costs. Additionally, where solid phase synthesis is used in the context of high-throughput screening applications (e.g. solid-phase combinatorial synthesis methods), the presence of even small amounts of impurities may create a false positive “hit” that misleads researchers, resulting in lost time, effort and money.
State of the art methods of living/controlled polymerization that generate polymer grafts on solid supports do not allow one to control the depth of penetration (i.e. in-growth) of grafts. Consequently, substrate polymers for solid-phase synthesis, made using state of the art methods, have unpredictable surface properties. Specifically, the transition (i.e. boundary layer) between the bulk polymer of the solid support and the polymer of the graft may vary considerably between different preparations. As described above, these differences may greatly affect the usefulness of a substrate polymer in solid-phase synthesis, or any other solid-phase application that requires consistent surface characteristics.
In accordance with the present invention, the inventors have developed more efficacious methods for controlled polymerization on solid supports which facilitate the production of a range of polymers grafted to a solid support. These improved methods allow polymerization to occur directly from the non-functionalized surface of the bulk support. In addition, these methods may employ the non-nitroxide-based RAFT and ATRP control agents that allow controlled polymerization to proceed at relatively low temperatures (<80° C.). Significantly, these improved methods may be controlled so as to provide reproducibly thin layers of graft polymers with decreased levels of “in-growth” of the bulk solid support.